1. Field of the Invention
The present invention relates to an ink jet recording apparatus that discharges ink, thereby performing a recording operation, and an ink jet recording head used in such a recording apparatus.
2. Description of the Related Art
Ink jet recording apparatuses can record color images at low operational cost and can be reduced in size, and therefore have been widely used, for example, in computing output devices and commercialized.
In recent years, in order to realize higher-speed and more detailed image recording, the realization of a recording head having a longer recording width (discharge port array length) has been hoped for. Specifically, a recording head having a length of 4 to 13 inches has been demanded.
The longer and higher-speed a recording head is, the larger the energy input into the recording head is, and the more significantly the temperature of the recording head during recording increases. This causes, for example, fluctuation in discharge amount per page, unstable discharge at high temperature, and deterioration in ability of continuous recording. Therefore, measures to maintain the recording reliability need to be taken.
Japanese Patent Laid-Open No. 8-150711 and U.S. Pat. No. 6,074,035 describe, for example, air-cooling from outside a recording head, and attaching a cooling tube to a recording head.
However, conventional ink jet recording heads have a problem such as that shown in FIG. 18. FIG. 18 shows the temperature distribution of a recording head having a coolant flow passage for circulating coolant provided along the discharge port array thereof just after the execution of recording. The horizontal axis shows the position in the direction of the array of recording elements (for example, heaters serving as electro-thermal transducers) of the recording head. The vertical axis shows the temperature in the vicinity of a recording element. It is proved by experiment that, in the vicinity of the end of an ink jet recording head, the temperature decreases toward the end.
It is also proved that, with the increase in the temperature in the vicinity of a recording element of an ink jet recording head, the amount of ink discharged increases. For example, the amount of ink discharged increases by 0.5 to 1.0 percent per degree rise in temperature.
Therefore, if the temperature of the end of a recording head is lower than that of the middle thereof, the amount of ink discharged from a discharge port at the end of the recording head is smaller than the amount of ink discharged from a discharge port in the middle of the recording head. As a result, when it is tried to form an image of uniform density on a recording medium, the density of the end of the resulting image is lower than the density of the middle of the image.
The recording head described in U.S. Pat. No. 6,074,035 eliminates the above-described temperature distribution by “providing a print head with a flow passage so that a liquid flows in the direction in which a distribution of temperature can occur, changing the cross-sectional area of the flow passage according to the temperature distribution that can occur in the liquid flowing through the flow passage, and thereby producing a distribution of flow rate.” For example, the cross-sectional area of the flow passage is largest on the most upstream side, decreases downstream, and is smallest and constant in the middle portion and the portion downstream thereof. By such a configuration, the above-described temperature distribution can be eliminated.
However, in order to eliminate the temperature distribution by this method, the cross-sectional area of the most upstream portion needs to be rather large, and this causes the recording head to be large.